1. Field of the Invention
The present invention relates to a charged particle beam inspection system, and more particularly to an apparatus and a method that reduce inspection image variation during inspection of a position near a substrate edge.
2. Description of Related Art
Fabrication of a semiconductor device, such as a logic or memory device, may include processing a substrate or wafer via any of various semiconductor processing tools. As feature size continuously shrinks, e.g., from 45 to 16 nm, use of the charged particle beam inspection tool becomes increasingly important for detecting abnormalities within the device at steps as early as possible. A conventional charged particle beam inspection tool 100 is shown in FIG. 1, which includes a charged particle emitter 110, a condenser lens 112, an aperture 114, an in-lens detector 120, an objective lens 130, and a substrate chuck (mechanical or electrical) assembly 170 mounted on an X-Y stage 270 to sustain a substrate 160. The imaging practice is irradiating a charged particle beam 102, to the surface of the substrate 160 and detecting the backscattered and secondary charged particles emanating from the surface of substrate 160. Detailed imaging practice is followed as that the primary charged particle beam 102 is emitted from the charged particle emitter 110, condensed by the condenser lens 112, and passed through the aperture 114. Then, the objective lens 130 will focus the primary charged particle beam 102 on to the surface of the substrate 160, and secondary charged particles are emanated from the surface of the substrate 160 to the in-lens detector 120. The objective lens 130 includes pole-piece 132, deflectors 136, and electrodes 138.
In order to perform a low energy inspection, a bias voltage is applied between the plate holding the substrate 160 and the objective lens 130. Since the substrate is not located in an area free of electrical fields, its material, shape and bias voltage may change the spatial electrical field distribution between the substrate 160 and objective lens 130. In general, the spatial electric field is substantially axial symmetrical with respect to the central beam, i.e., the central axis 102 of the assembly. This is essential for the charged particle beam to be focused into a small spot of round shape. However, when the edge peripheral portion of the substrate 160 is moved close to the central axis 102, the spatial electrical field distribution will become significantly non-axial symmetrical due to the bias voltage on substrate 160, discontinuity of the material and shape outside the substrate 160 and stage 170. The non-axial symmetrical distributed electric field shifts the incident primary charged particle beam, and will cause the obtained image varying from the expected in position, scaling, and focus, known as image shifting, distortion and defocus.
One conventional chuck assembly 170 for sustaining a substrate 160 is shown in FIG. 2A and FIG. 2B, wherein the chuck assembly 170 has a peripheral protruding region 172 to enclose the substrate 160. A small gap 174, which is most likely not equal around the chuck, is designed for tolerating the position accuracy when the substrate is loaded. Therefore, the gap will not form uniform electric field and the edge field effect will distort the primary beam due to the gap. In order to solve the edge effect issue, Mankos et al., as described in U.S. Pat. No. 6,903,338, provide a method to wear out the edge effect, as shown in FIG. 2B, wherein the gap 174 is increased for adopting an insert ring 180 and a voltage is applied to the insert ring 180 according to size of the gap 174 to reduce the edge effect during the inspection of the substrate edge.
However, the prior art of enlarged gap with inserted and biased ring 180 by Mankos et al. is impractical to implement. The height, shape and flatness of the ring 180 is too difficult to control by machining and assembly, and it also needs additional isolation inside the small gap from holder base 270 and wafer 160, where charged particle beam, either primary or secondary, is attached to the isolation, the insulation performance and charging effect may induce more severe problems to the tool.
The present invention provides another apparatus and method to reduce the substrate edge effect during patterned substrate inspection.